1. Field of the Invention
The invention relates to a ball bearing.
2. Description of the Related Art
Ball bearings are subjected to a lower torque (lower rotational resistance) than roller bearings such as tapered roller bearings. However, in recent years, there has been a demand for a further reduction in the torque exerted on the ball bearing. The ball bearing has an inner ring, an outer ring, a plurality of balls, and an annular cage. The cage functions as a separator that separates the balls provided between the inner ring and the outer ring from one another. The cage reduces the rotational resistance to the bearing and restrains the balls from being worn away.
As depicted in FIG. 8, a cage 90 for a ball bearing has an annular portion 91 and a plurality of cage bars 92. The annular portion 91 is provided on an axially first side of balls 93. The cage bars 92 are provided so as to extend from the annular portion 91 toward an axially second side. Pockets 94 in which the balls 93 are housed correspond to areas each located on the axially second side of the annular portion 91 and between a pair of the cage bars 92, 92 adjacent to each other in a circumferential direction.
Conventional ball bearings adopt a configuration referred to as a rolling element guide and in which the cage 90 is guided by the balls 93. In other words, the cage 90 is positioned by the balls 93. To implement the rolling element guide, each of the pockets 94 has a pocket surface 95 shaped along a virtual spherical surface having a slightly larger diameter than each of the balls 93. To prevent the cage 90 from coming off, each of the cage bars 92 has a pawl portion 97 that further protrudes toward the axially second side. An inner surface 96 of each of the pawl portions 97 is included in the pocket surface 95, shaped along the spherical surface. (See Japanese Patent Application Publication No. 2014-70669 (JP 2014-70669 A).
In order to allow the rolling element guide configuration to be adopted, the cage 90 needs to encase the balls 93. To achieve this, each pocket surface 95 is shaped along the virtual spherical surface having a slightly larger diameter than the corresponding ball 93 as described above. The pawl portions 97 protrude far from the respective cage bars 92 toward the axially second side. This configuration results in an increased size of the cage 90 and an increased area of each pocket surface 95 facing the corresponding ball 93.
Rotation of the ball bearing causes shearing of grease present between each ball 93 and the corresponding pocket surface 95 and around the ball 93 and the pocket surface 95. An increased area of the pocket surface 95 facing the ball 93 as described above leads to an increase in resistance resulting from the shearing (shearing resistance). Thus, disadvantageously, the ball bearing involves high rotational resistance (running torque) and a reduced lifetime of the grease.